Uv Sanitation For Indoor Pool

[tazcat]

Hi All,

Just joined the Forum, but been reading here for a while. I have a new, 7000 Gal, fiberglass pool installed indoors and was looking into alternate sanitation methods to minimize chemical use and ongoing expense. I'm seriously considering an Ultraviolet unit to acheive this goal, with the understanding that a small chlorine residual would probably be needed, but much less than normal. My plan is to run a small circulation pump (75 Watts) 24/7 to keep the water flowing through the UV unit (also 75 Watts) at about 15 GPM (total cost about $10 per month in the New England area).

Has anybody had experience with UV units in residential pools? I know they are big in the olympic pools, and certainly at waste-water treatment plants, but I'm wondering if anybody can attest to their effectiveness in reducing chemical usage in a residential setting.

Thanks, Dave

[waterbear]

It is a fallicy that a smaller chlorine residual is needed with UV or ozone. Neither has any residual action and you still need a fast acting residual sanitzer in the water. Just because you are using a non residual sanitizer it does not mean the LEVEL of residual chlorine you need is less, it means that you will need less chlorine to achieve that residual.

The main advantage to using UV in an indoor pool is that it will help with eliminating chorarmines and some of the more noxious oxidation byproducts.

[tazcat]

Thanks Waterbear. You are correct in that I intend on reducing the amount of chlorine needed to maintain the FC level, along with the chloramine reduction given the indoor setting.

Also, given a low bather-load of maybe one or swimmers once a day would you expect that 0.5 to 1.0 ppm FC would be sufficent?

[waterbear]

Also, given a low bather-load of maybe one or swimmers once a day would you expect that 0.5 to 1.0 ppm FC would be sufficent?

It depends on whether you intend to add some CYA to your water or not. There is growing eveidence that a small amout of CYA (around 20 ppm) can help minimize volitile oxidation byproducts and persistant CC that plague indoor pool. It is not an accepted practice in the industry but there are many things that work RIGHT that are not 'accepted' by the industry.

[tazcat]

Yes, I've read that here about the CYA for indoor pools. So let's say I keep 20 ppm CYA, would level of FC would I need to maintain. I'm trying to keep it simple, so I'm just using 12% bleach added to the pool as needed; can the CYA be added in a similar manner? Any down-side to having the CYA in an indoor pool. Also, don't know what effect this has, if any, but the water temperature is maintained at about 85 +/- degeees F (my wife likes it warm....)

Thanks, Dave

[chem geek]

Dave,

I'd have the FC at a target of 4 ppm if you have 20 ppm CYA in the pool. This is a somewhat higher active chlorine level than found in outdoor pools, but you don't have UV to aid in any oxidation of organics and even with 4 ppm FC and 20 ppm CYA this is technically equivalent in oxidizing/sanitizing power to a pool with around 0.2 ppm FC and no CYA so is still quite low.

The nitrogen trichloride production is theoretically lower by a factor of 10 or more, but the monochloramine and dichloramine intermediate levels until fully oxidized will be that same factor higher. These latter two are far less volatile and irritating than nitrogen trichloride so the tradeoff is reasonable. The oxidation of other organics in the pool will also be slower as will the oxidation of your skin, swimsuits and hair. The outgassing of chlorine itself should also be about a factor of 10 lower. In theory, your loss of chlorine from the UV system should be lower as well.

As for adding CYA, you can do so slowly using pure CYA or you can use Dichlor as your source of chlorine for a while. For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm. So if you use Dichlor over time to cumulatively add around 22 ppm FC, then you'll be at 20 ppm CYA.

The temperature at 85F isn't a huge factor -- when CYA is present, it slightly increases the active chlorine concentration relative to lower temperatures. The temperature differences are more important at higher spa temperatures (i.e. 100-104F).

Richard

[tazcat]

Thanks Richard, although I think I'm a little confused now. It seems like adding the CYA reduces the effectiveness of the chlorine, increases the chloramines, but reduces the nitrogen trichloride. Not sure what the nitrogen trichloride is, but I know the chloramines are bad and so I guess the nitrogen trichloride must be worse?

Also, is does the UV from the sun affect the chemistry differently than would a UV system (since it only has UV-C)?

I read in another post you responded to regarding the difference between FC and active chlorine and how it affects bather comfort, and that's part of my confusion - why not just maintain a very low FC level without any CYA? I'm not questioning the validity, just trying to put it all together.

Dave

[chem geek]

Dave,

Sorry to confuse you. When I reread my post, I see where it is confusing. The UV from the sun is weaker, though broader in spectrum, then the UV system you are using. The UV in sunlight tends to keep nitrogen trichloride in check since it's very sensitive to UV, but it's also very volatile and by far the most irritating component in even small quantities. So though your UV lamp would remove nitrogen trichloride from the water, odds are it would outgas rather than get to your UV lamp.

The monochloramine and dichlormaine are far less irritating by comparison and they are also less volatile. Nevertheless, if you used a whole lot of CYA, then you'd have too many of these two chloramines. So a balance is what is desired -- a little CYA is a good thing, having none or having too much isn't. Of course, you could accomplish the same thing by having a very low 0.2 ppm FC level in the pool and that's pretty much what is done in Germany with 0.2 to 0.6 ppm FC and no CYA, but it's hard to control/regulate and bather load can wipe that out locally.

So think of the CYA as a chlorine buffer that lets you have a higher FC "buffer" available for even higher bather loads (so the chlorine doesn't "run out" locally), but moderates the chlorine strength so that it isn't too strong.

Richard

[tazcat]

Richard,

Thanks for the clarification - I think your last paragraph summed it up well for me.

Also, since I keep the pool covered with a bubble-type cover (to retain the heat and control humidity) when not in use, would the outgassing of the nitrogen trichloride be minimized due to the near zero surface area, and therefore have a better chance of being eaten up by the UV system?

Dave

[chem geek]

Dave,

The cover will probably keep much of the nitrogen trichloride from escaping so yes that gives the UV system a chance to break some of it down more readily.

I also want to expand a little bit on what I wrote. Technically, the amount of monochloramine produced will be related to how much ammonia is introduced from sweat and urine. When I said it would be higher with more CYA in the water, I'm referring to an average amount assuming continual introduction of ammonia into the water. If there is a one-time introduction, then the amount of CYA doesn't matter with respect to monochloramine (though it does with respect to dichloramine and nitrogen trichloride).

Also, urea is the largest component of sweat and urine (after water, of course) and there is no definitive model for chlorine oxidation of urea -- pretty amazing that no one has done this yet. There is some speculation that after a quadchlorourea is formed, additional chlorine breaks this down to dichloramine and nitrogen trichloride. My statements regarding CYA's effects on the various levels of chloramines still applies. I write about the chemical details of all this here.

Richard

[tazcat]

Richard,

Thanks for all your help. My background is in electrical engineering, so I need all the help I can get on the chemical side, and you 've certainly been a great resource!

Dave